There has been known an LED array having a plurality of light emitting diodes (abbreviated as LED) arranged therein as a light emitting device which is applied to an optical printer head of an electrophotographic printer. The LED array includes a plurality of bonding pads to individually connect the light emitting diodes to a driving circuit. For example, in the case where an electrophotographic printer is configured with the specification of A3 size and 600 dpi (dot per inch) and anodes or cathodes of the LEDs are made to be a common electrode realized by a conductive substrate, the same number of connecting portions between bonding pads and circuit wiring lines as the number of light emitting elements is required, which amounts to about 7,300 portions. Accordingly, a very long time is required for bonding the both by a wire bonding method, which makes it difficult to improve the productivity. Furthermore, a larger area than that required for forming the light emitting elements is required to form the bonding pads and the number of light emitting elements per unit length in the scanning direction increases as an image to be formed by the electrophotographic printer increases in precision, which causes an increase of the number of bonding pads.
There is known as a first conventional technique for reducing the number of bonding pads, a light emitting element array employing a dynamic (time-division) driving scheme. In the technique, an LED array is divided into n1 groups including m1 LEDs (wherein n1 is a positive integer and m1 is a positive integer), the anodes or cathodes of the LEDs of the groups are formed as a common electrode, and m1×n1 matrix wiring lines are made thereon. In a dynamic (time-division) driving operation, a driving signal applied to the matrix wiring lines is time-divisionally switched to allow the LEDs to emit light. By using the dynamic driving LED array, the number of bonding pads can be reduced to about ¼, compared with the above-mentioned LED array in which the LEDs are connected to the driving circuits, respectively (for example, Japanese Unexamined Patent Publication JP-A 11-268333 (1999)).
There is known as a second conventional technique, a dynamic driving light emitting device for time-divisionally driving a light emitting element array in which field effect transistors are connected to the LEDs, respectively (for example, see Japanese Unexamined Patent Publication JP-A 6-177431 (1994)). In the light emitting device, a driving IC (Integrated Circuit) having switching elements such as NAND gates built therein is connected to the light emitting element array, the switching elements built in the driving IC calculate a logical product of a strobe signal (STB) and a gate signal, and the gate signal is outputted only when the strobe signal has a true value, whereby the light emitting element array can be driven dynamically.
There is known as a third conventional technique, in order to reduce the occupied area of the wiring lines connected to the light emitting elements, a light emitting element array in which an emission thyristor having a PNPN structure is used as the light emitting element, ones of the anodes and the cathodes are formed of a conductive substrate in common, and the other of the anodes and the cathodes is connected to the gate electrodes in a matrix (for example, see Japanese Examined Patent Publication JP-B2 2807910 and Japanese Unexamined Patent Publication JP-A 2001-217457). By connecting the gate electrodes, in which current hardly flows, all over the light emitting element array by the use of electrode wiring lines, it is possible to reduce the line width of the electrode wiring lines and to reduce the area required for forming the electrode wiring lines.
However, in the first conventional technique, since the electrode wiring lines are connected to the anodes or cathodes of the LEDs, main current proportional to the light emission intensity of the LEDs flows in any electrode wiring line. In this case, when the wiring line resistance is high, the power consumption of the driving IC is enhanced or the driving performance is deteriorated due to the wiring line resistance loss. Accordingly, the line width needs to be enhanced to reduce the wiring line resistance. Therefore, there is a problem in that the area required for forming the electrode wiring lines increases and the surface area of a chip having the LED array formed therein also increases.
In the first to third conventional techniques, for example, the dynamic (time-division) driving operation is performed using the m2×n2 matrix wiring lines, m2×n2 electrode wiring lines are required for one light emitting element array (wherein m2 and n2 are positive integers). However, when a light emitting device is configured to include a plurality of p2 light emitting element arrays, p2×(m2+n2) electrode wiring lines proportional to the number of light emitting element arrays are required (wherein p2 is an integer equal to or greater than 2). The number of output terminals of the driving IC to drive the light emitting element arrays needs to be enhanced depending on the number of required electrode wiring lines. When the number of terminals of the driving IC is equal to the number of terminals of one light emitting element array, the driving ICs corresponding to the number of light emitting element arrays are required. In this way, when the light emitting device is configured to include the plurality of light emitting element arrays, a lot of driving ICs are required for the conventional techniques and the number of wiring lines to connect the light emitting element arrays to the driving ICs is enhanced, thereby complicating the device or enlarging the device.
When the light emitting elements are arranged with a high density to obtain a high definition image, the number of bonding pads increases accordingly in the conventional techniques. However, since the pad pitch is too small, it is difficult to perform the wire bonding process. As a result, the increase in density of the light emitting elements is restricted.
In the second conventional technique, the driving IC having the switching elements such as NAND gates built therein needs to be connected to the light emitting element array. When a light emitting device is configured to include the plurality of light emitting element arrays, the number of driving ICs connected to the light emitting element arrays increases with the increase of the number of light emitting element arrays, thereby complicating or enlarging the device as a whole.